SAP Data Archive & Decommissioning.

I have a complex requirement and never done this before, I been to many online forums & blogs but still confused about start-up point. Kindly help me with this:

We have 2 locations where SAP is installed Site-A & Site-B. We want to merge Site-B into Site-A and Archive all data for Site-B and Decommission later on. Please share you experience, how to plan this project and what cautions we shall be take care about. What mandatory things we shall focus.